This invention relates to microwave ovens particularly, but not exclusively, to industrial ovens for food products such as biscuits, snacks, chips, meat substitutes. The invention may however be applied to non-food products, such as to the heating of plastics.
Continuous ovens for food products such as biscuits can be designed to have a plurality of zones through which each food product travels in sequence. Such zones can be used to provide different heating and baking conditions. Multi-media ovens combine different means of heating a food product in the different zones, such as cyclotherm radiant heating, gas or electrically heated convection, near infrared radiant heat, and are well-known in the industry.
In addition to the usual methods of heating we have shown it to be advantageous to inject microwave energy into an oven at various positions along the oven length. Such an oven which employs microwave heating in addition to another form of heating is hereinafter referred to as a `combined oven`.
Microwave heating provides means to induce a rapid transfer of energy to the product, the level of energy transfer being selected to provide a desired effect within the product. In a proposed combined microwave-biscuit oven, using four microwave zones, the first microwave zone induces a rapid rise in temperature within the product, the second microwave zone enhances development, and the third and fourth microwave zones reduce the moisture content of the product prior to leaving the oven.
Conventionally heated industrial ovens for food products generally use a metal band to support and convey the food products through the oven. The band may be a metal strip or a mesh band.
The use of a metal band in a multi-mode microwave oven, however, gives rise to serious problems. The microwave heating efficiency is found to be reduced to unacceptably low levels, especially with thin food products such as biscuits.
Industrial microwave units basically consist of a microwave generator and a microwave applicator.
We have appreciated that one of the reasons for the low efficiency of prior art combined ovens is that the conventional applicators operate in a plurality of modes, and that the use of substantially a single mode can provide advantages.
The terms `band-parallel` and `band-normal` used herein are intended to refer to directions which are parallel to and normal to, respectively, the plane of the oven band. Whilst the oven band will often be horizontal, it should be appreciated that the oven band need not always be horizontal since products can be conveyed on a sloping band.